In order to ensure the fitting of doors and windows during the construction (or reconstruction) of building walls, it is usual to make a larger opening than required for the door or window jamb (frame) thereby leaving an airspace between the jamb and the internal wall studs and insulation (if any). This airspace undesirably provides an air/vapour passageway from the outside of the building to the inside of the building. Therefore, while the airspace is necessary for the installation of the door (or window) it provides an insulation problem once the door has been installed.
In accordance with modern construction practices, it is also usual to apply an air-vapour barrier sheet, in the form of a plastic sheet (e.g. polyethylene) over the wall studs (and insulation) and below the interior wall cladding (e.g. gypsum board) to prevent outside air and moisture from passing through the wall to the cladding and the interior of the building and to prevent interior moisture and air from passing into the wall cavity and outside. Such vapour-barrier- sheet-.may be stapled to the wall studs to secure it during the construction phase. Since it is awkward to affix the air-vapour barrier sheet to a window (or door) jamb, it is ordinarily only run over the length of the wall studs/insulation to the start of the airspace thereby leaving a gap across the airspace through which air and vapour may pass. Consequently, it is desirable to provide a means by which the airspace between a window or door jamb and the interior wall studs/insulation and air-vapour barrier sheet (if any) may isolate the exterior from the interior of the building.